1. Field of the Invention
The present invention relates to apparatus for use in a mobile radio system, and more particularly it relates to the control of transmit power from a mobile unit to a base station. The invention is applicable to CDMA systems, but is not necessarily limited thereto.
2. Description of the Related Art
The efficient operation of a CDMA cellular mobile radio system requires that every mobile unit transmit the minimum power necessary for acceptable communication at all times. In a frequency re-use environment with many base stations this also implies that the mobile unit must transmit at all times to the base station permitting operation with the minimum mobile transmit power. The choice of this base station will be a function, not only of the relative path losses, but also of the relative interference levels at the two (or more) preferred base stations.
A known solution for solving this problem for the uplink (mobile to base station direction) has been the use of a so-called `soft handoff`. In this mode of operation, two (or more) base stations attempt to receive the signal and demodulate it. These base stations then pass their signals onto the mobile switching center where the better signal is selected.
This approach has two problems. During operation of the `soft handoff` mode, two (or the number of base stations involved, if greater) simultaneous links must be established. There is a cost associated with this additional traffic, particularly if the backbone network capacity is rented from a third party operator.
Furthermore, the use of CDMA cellular mobile radio requires fast accurate power control to mitigate the so-called `near far problem` which is known to those versed in the art. If closed loop power control is implemented, there is a potential conflict between the control exerted from the two base stations. One known approach implements a `lowest wins` strategy in which the control of one mobile unit by two base stations is exerted in such a way that the mobile unit will increase its power only if both base stations require this. There is a potential inefficiency involved in this strategy which is illustrated by reference to FIG. 1.
Referring to FIG. 1, the first plot shows the path gain relative to power control demand threshold for two base stations receiving a signal from one mobile unit. Taking each base station individually, then the position would be that whenever the relative path gain equals the threshold, the mobile transmitter is set to a nominal value. Otherwise it is increased or decreased as necessary to keep the received signal at the threshold. The `lowest wins` strategy leads to the illustrated power profile. This in turn leads to the conditions illustrated in the second and third plots. It can be seen that in neither case is the threshold power obtained over the entire period. If this period corresponds to an error control coding interleaving frame, then it is likely that the signal may be uncorrectable at both base stations. To avoid this, it is necessary to increase the power control threshold. However, this would result in an unnecessary increase in transmit power whenever only one of the base stations was dominant throughout a frame. Moreover, the added power fluctuations tend to degrade error performance.
An aim of the present invention is to provide means by which the above mentioned problems are avoided while retaining the necessary responsiveness to path changes.